One kind of antiskid control device is disclosed in U.S. Pat. No. 4,912,641. This device comprises wheel brakes for applying braking force to wheels of a vehicle, a master cylinder for generating brake fluid pressure corresponding to the extent to which a brake pedal is depressed and for transmitting the brake fluid pressure to the wheel brakes, brake fluid pressure control valves interposed between the wheel brakes and the master cylinder, wheel speed detection devices for detecting the rotational speeds of the wheels, and a controller for operating the brake fluid pressure control valves so that the brake fluid pressure is reduced and duty pressure is increased in response to output signals from the wheel speed detection devices.
This control device is designed to be capable of detecting the condition in which the wheels tend to be locked before starting the antiskid control and repeating the control for reducing the pressure and increasing the duty pressure to prevent the wheels from being locked. With this device, when it is determined that the duty pressure should be increased, the duty pressure increase operation is executed repeatedly (the duty pressure increase process comprises the duration of duty pressure increase and the duration of pressure reduction, and the sum of them is constant), and the duty ratio (the ratio of the duration of duty pressure increase to the duration of pressure reduction) is set so that the duration of the duty pressure increase becomes gradually longer. Furthermore, when the necessity of pressure reduction is determined after the duty pressure increase control has been executed, the initial value of the duty ratio in the next duty pressure increase control is increased or decreased depending on a period or a duration of the preceding duty pressure increase control so that the pressure increase gradient can be properly compensated.
Such a conventional device, however, is not designed so that the duty ratio for controlling the duty pressure increase can be varied according to the difference between the braking pressure generated by the master cylinder (hereinafter referred to as M/C pressure) and a locking pressure which causes the wheels to be locked.
Thus, in the situation where the duty ratio is set for a relatively short duration of pressure increase, when the M/C pressure is at a high level (or when the difference between M/C pressure and the locking pressure is relatively large), there is a large difference between the M/C pressure and the braking pressure of the wheels, and thus a necessary pressure increasing gradient can be obtained. However, when the M/C pressure is close to the locking pressure (or when the difference between the M/C pressure and the locking pressure is relatively small), the difference between the M/C pressure and the braking pressure applied to the wheels also becomes smaller. Thus, when the duty ratio is left unmodified, the pressure increase gradient becomes too gentle, thereby giving rise to problems such as pressure increase delay or a longer braking distance.
On the other hand, in the case where the duty ratio is set so that the duration of pressure increase becomes longer, when the M/C pressure is close to the locking pressure, the necessary pressure increase gradient can be obtained, but if the M/C pressure is too high, the pressure increase gradient becomes too steep. Consequently, the wheels become more likely to be locked as a resultant problem.